Recent technological advances in the semiconductor industry have permitted dramatic increases in circuit density and complexity, and commensurate decreases in power consumption and package sizes for integrated circuit devices. Single-chip microprocessors now include many millions of transistors operating at speeds of hundreds of millions of instructions per second to be packaged in relatively small, air-cooled semiconductor device packages. A byproduct of these technological advances has been an increased demand for semiconductor-based products, as well as increased demand for these products to be fast, reliable, flexible to manufacture and inexpensive. These and other demands have led to increased pressure to manufacture a large number of semiconductor devices at a rapid pace while increasing the complexity and improving the reliability of the devices.
Integrated circuit fabrication relies heavily on metrology tools that can provide accurate and reliable information related to the dimensions of a variety of structures therein. For example, it is often desirable to obtain the dimensions of active and passive device structures in an integrated circuit, such as interconnects that link electronic transistors and other circuitry in the integrated circuit. Optical techniques that have previously been available for the measurement of structural dimensions typically rely on the interference of light beams reflected from the front and back surface of the structure being measured, and have typically been limited to structures that pass light (e.g., dielectric structures. However, these approaches have not typically been suitable for use with metal structures. Due to the high conductivity of metal structures, the penetration depth of light is usually much less than the metal structure dimensions (i.e., thickness, width or length), with the metal structure often acting like a mirror. In particular, the penetration depth of light in copper varies from about 0.6 nm at the ultraviolet wavelengths (λ=100 nm) to about 6 nm in the far infrared (λ=10 μm). For this reason, a common optical reflection measurement does not yield information relative to certain dimensions of the structure to be measured.
These and other limitations discussed herein have been a challenge in the measurement of structures and in particular to the measurement of various dimensions of structures used in the semiconductor industry.